Testing an expression with the primitive if

As in all programming language, Logo allows you to check if a condition is satisfied and then to execute the desired code if it's true or false.
With the primitive if you can realize those tests. Here is the syntax:

if expression_test [ list1 ] [ list2 ]

if expression_test is true, the instructions included in list1 are executed. Else, if expression_test is false, the instructions in list2 are executed. This second list is optional.
Examples:

• if 1+2=3[print "true][print "false]
• if (first "XLOGO)="Y [fd 100 rt 90] [pr [ XLOGO starts with a X!] ]
• if (3*4)=6+6 [pr 12]

Important: When the result of the first predicate is equal to false, the primitive if looks for a second list, I mean an expression starting with a square bracket. In some very rare cases, it can't be done, and you'll have to use the primitive ifelse . For example:

# We  affect two lists in variables a and b
 make "a [print true]
 make "b [print false]

# First test with the primitive if--> The second list can't be evaluated.
 if 1=2 :a :b 
 What to do with [print false]?

# Second test with the primitive ifelse --> success.
 ifelse 1=2 :a :b
 false